This invention relates to a voltage integrating measuring system for parametric ratio control and more particularly to a data acquisition system for use in computer control of aluminum smelters.
At the present time aluminum is commonly produced by passing electric current through a cell or "pot" containing an electrolyte bath comprising essentially alumina and cryolite. The current flows between a moveable anode and a stationary cathode formed by the carbon lined steel shell, having above the carbon the molten aluminum covered by the cryolite. A large number of "pots" (e.g. 150 or more) are electrically connected in series to form a potline.
Alumina is fed periodically to the cryolite bath to replenish the consumed amount, which has been converted to aluminum and oxygen. Without firm knowledge of the actual concentration it is hard to control the required rate of feed and avoid the danger of overfeed. Traditionally, "anode effects" have been used as low concentration check-points to avoid overfeed. With the necessity of automation and a variety of automated feed systems the control of the concentration became an essential part of successful operation.
The current method to control the individual "pots" uses the established formula to obtain "R"--the resistance of the electrolyte cell: ##EQU1## Where: V=the total cell potential,
Vb=the assumed value of the back e.m.f., PA1 I=the potline current.
The pot potentials (V) are sequentially measured for each pot simultaneously with the pertaining potline current (I). The assumed back e.m.f. (Vb) is subtracted from the total potential (V) and divided by the measured current (I). A batch of about four such measurements per five minute period are stored in a computer memory for later conversion. Since the Vb value is an assumed, or a statistically derived one, the individual confidence limit is low.
A large smelter with several potlines may have several hundreds of cells so that the gathering sequentially of the required voltage and current data is a complex and time consuming process. The ratio between the individual measuring time and the total cycle time is in the order of 1/10,000--insufficient to obtain the narrow measuring range required to monitor the depletion of the concentration. Further, it requires the use of a large number of measuring cables, complex scanning units and expensive measuring units. The known scanning systems limit the amount of data which can be acquired within the available time. They cannot follow the natural low frequency components of the cell voltage, and the measured data, being out of rhythm, is inaccurate.